From among the large range of valves for flows commanded by pressure differences, which work by transferring control flows and which may regulate flows, the following documents of patents may be mentioned: U.S. Pat. No. 5,738,332 and U.S. Pat. No. 5,769,387 by S. Pérez, the same inventor of this device.
S. Pérez's U.S. Pat. No. 5,738,332 patent discloses a main valve with a flow regulating device consisting in a secondary control valve, which may be a valve of any state of the art and allowing a preset regulation of the flow between a minimum closed position and a maximum opened position. These secondary control valves allow a restricted preset regulation of the flow of the main valve, since for setting any flow for the main valve, the secondary control valve should be operated, thus gradually defining a new flow from the initially regulated flow up to the new flow required. In addition, these devices do not allow operating the main valve from pressures which are slightly over to zero pressure, because the restitution force of the main valve's shutter is to be overcome, especially when pressure in the system is low. Patent U.S. Pat. No. 5,769,387 by S. Pérez, discloses a main valve where regulating and/or controlling devices may be installed, which are arranged at the inlet of control flow, at the outlet of the control flow or both. These regulating and/or controlling any flow at the main valve, the control or regulation devices should be operated in order to define a new flow gradually from the flow initially regulated to the new flow required. In addition, these devices do not allow operating the main valve from low differences of pressure between the inlet and the outlet of the main valve, which may determine control forces which are not enough to overcome the restitution force of the main valve's shutter, while maintaining in these cases the main valve closed.